2025MNRAS.540.2965H

We present, for the first time, an in-depth dynamical analysis of the spectrotemporal properties of the soft variability classes ($\delta$, $\kappa$, $\omega$, and $\gamma$) of GRS 1915+105 during the detection of $\sim$70 Hz High-Frequency Quasi-periodic Oscillations (HFQPOs) using AstroSat data. The wide-band spectra ($0.7-50$ keV) are well described by thermal Comptonization along with an extended power-law component. Additionally, power spectra ($0.01-500$ Hz) indicate that Comptonized photons ($6-25$ keV) primarily contribute to the HFQPOs. Our findings reveal that high (low) count rates referred to as 'non-dips' ('dips') in the light curves of the variability classes correspond to the detection (non-detection) of HFQPOs. Accumulated 'non-dips' ('dips') spectra are modelled separately using thermal Comptonization (nthComp) as well as kerrd which indicates harder spectra and smaller inner disc radius during the detection of HFQPOs. We conduct dynamical analyses (every 32 s) to trace the presence of HFQPOs, and variations in thermal Comptonization parameters ($\Gamma _{\rm nth}$ and ${\rm N}_{\rm nth}$). Moreover, we observe a positive correlation of 'non-dips' with QPO strength, ${\rm HR}1$, and ${\rm N}_{\rm nth}$, while $\Gamma _{\rm nth}$ shows an anticorrelation, suggesting that high-energy photons from the Comptonized corona are responsible for the HFQPOs. Furthermore, we estimate the size of the Comptonized corona using kerrd and diskpn to be $\sim 2.8 - 16$ $r_{\rm g}$. Thus, we infer that a 'compact' oscillating corona likely modulates the high-energy radiation, exhibiting the 70 Hz HFQPOs in GRS 1915$+$105.